Make num dependency optional

sexplib pulls in a dependency on `num` which for most users is
unnecessary. Those users that need `sexplib.num` to convert to and from
`num` values have a dependency on `num` already, those who don't have no
need for `num` in their dependency cone.

num/lib/dune

@@ -2,4 +2,5 @@
  (name sexplib_num)
  (public_name sexplib.num)
  (libraries num sexplib)
- (preprocess no_preprocessing))
+ (preprocess no_preprocessing)
+ (optional))

sexplib.opam

@@ -14,6 +14,8 @@ depends: [
   "parsexp"
   "sexplib0"
   "dune"     {>= "3.11.0"}
+]
+depopts: [
   "num"
 ]
 available: arch != "arm32" & arch != "x86_32"